This invention relates to a process for producing cadmium stannate layers in which cadmium stannate layers are applied onto a glass substrate by a dipping process, as well as to suitable solutions for implementing the dipping process.
Cadmium stannate layers on glass surfaces are known and characterised by a high electrical conductivity, high infra-red reflecting properties and a low adsorption within the visible spectrum. The application areas offered to the technician can be clearly determined from this, and only a few of them are illustrated here, e.g. heated windows, windows for buildings, doors for microwave ovens, solar collector panels, limiting surfaces for fluid crystal display, etc.
Cadmium stannate layers have been produced to date in a satisfactory quality by means of high frequency atomizing. The process is slow in making up layer thickness and expensive from a capital investment viewpoint. The coating of large area constructions such as window panes is hardly economically rational, since the homogeneous covering of large surfaces requires an enormous expenditure in capital equipment.
The spray process has been described in principle as a second possibility. An aqueous solution of cadmium and tin salts, to which other salts are added in the form of process additives, is sprayed upon hot glass (viz. 600.degree.-700.degree. C.). Depending on the temperature, there is formation of CdSnO.sub.3 or Cd.sub.2 SnO.sub.4, as well as CdO and SnO.sub.2, through not as a uniform layer corresponding to the chemical composition of the solution. This is clearly outlined, inter alia, in DT-OS No. 26 54 094 where, according to claims 2 and 3, a molecular ratio of Cd:Sn=3:1 to 40:1, preferably 20:1 is indicated for the solution when seeking a ratio of Cd:Sn of 1:1 or 1:2 in the layer.
Circumstances are even more complex on transition to a multi-component system, for instance when additives are added to the Cd and Sn salt. This is also applicable in the case of high frequency atomizing, as described in AD-A-008-783 NTJS. Quite frequently, an additive for increasing the electrical conductivity, such as indium, does not integrate in the cadmium stannate lattice (at least not in the same quantity as added) and thus remains with little or no effect. The atomizing process furthermore involves a high loss of expensive spray solution, which is not transformed into the covering layer. The environmental pollution arising from toxic cadmium, and its eventual expensive removal, conceals a number of other extremely thorny problems.
Success of the layer formation depends to a considerable extent upon the temperature employed in the atomizing process. In the atomizing of individual heated windows, the edges and the center having differing temperatures during the course of layer formation, leading to a lack of uniformity in the layer in the form of differing thicknesses and differing chemical compositions within the layer. This is particularly evident when the layer thickness approaches an area subject to obvious interferences, which is most common for such layers. When layer atomizing on a moving glass strip, conditions are somewhat improved in relation to individual windows, though the consequences already outlined still remain serious. In addition, even the best nozzle optimization will not ensure a fully uniform coverage with minimal droplets of the closest droplet size distribution and the smallest distribution of kinetic energy in the droplets.
Recourse to ultrasonic atomisation of the spray medium requires the use of a carrier gas to transport the mist to the glass surface, which is the case of internal coating to a tube (which is the most favourable geometry) admittedly results in a much improved homogeneity, though not in the case of large areas of glass, e.g. measuring several square meters. Furthermore, the growth rate of the layer falls off, since the coating material must be continuously transported towards the cooling substrate and consequently has to be reheated over extended periods.